Universal motors have widely been used as for high-speed rotation electric blowers. This type of motor has a brush of short lifespan because of wear due to high-speed rotation exceeding 50,000 RPM, high-voltage driving of 200V, or high-capacity operation reaching 2 Kw. Accordingly, a semiconductor-controlled synchronous motor, such as a three-phase brushless DC motor, has recently been made available. The semiconductor-controlled synchronous motor has a relatively longer lifespan. However, because a permanent magnet is used for a rotor, it is apt to be damaged by the centrifugal force during high-speed operation.
Japanese Patent No. 2823817 teaches a permanent magnet embedded motor, in which a magnet is buried in an armature iron core in an attempt to strengthen the rotor. However, the achieved strength is still insufficient.
Conventionally, a three-phase inverter circuit has been used, which uses a Hall IC in detecting rotational position, and a distribution IC as a driving control circuit. A stator iron core is formed with a plurality of slots in its inner circumference for wire distribution, but has a structure which does not allow cool air to easily pass through the gap between the rotor and the stator. Accordingly, the temperature of the stator increases, thereby affecting efficiency. Additionally, a smoothing condenser of rectified voltage is required for the driving control circuit, occupying an unnecessarily large area. The above also has increased costs.
When employed in high-speed rotation, such a synchronous motor has insufficient resistance against centrifugal force due to its structure in which the rotor uses a permanent magnet. Furthermore, because the driving control circuit of the brushless DC motor is complicated, a large space is required for accommodating the circuit which does not meet with the current trend for compact and economic products.
Additionally, because there is almost no cool air flowing in the gap between the stator and the rotor, motor and blowing efficiency deteriorate. If the number of power switching elements is reduced, starter torque deteriorates, and the motor may not start instantly, or may have an abnormality. Superchargers for automobiles particularly have deteriorated efficiency in starting the operation, as the superchargers use backpressure of the engine.